Digital SLR Crop Factor – Boon or Bane?

There’s a lot of confusion when it comes to using lenses designed for 35mm cameras on the current crop of Digital SLRS. Many photographers have sizeable investments in 35mm lenses for their film cameras, and for the most part their lens investments drive them towards a particular digital camera body, whether it’s Nikon, Canon, Minolta (Sony), Pentax or Olympus.

Luckily, all the manufacturers retained their old AF mounts, except for Olympus, which uses the newly designed Four-Thirds mount. Sony retained the Minolta ‘A’ autofocus bayonet mount for the Alpha series cameras, allowing all the fine 35mm lenses out there since 1986 to be used. The same goes for Nikon and others.

However, due to cost constraints at the present time, most digital SLRs use a sensor that is smaller than the 35mm format size of 36mm x 24mm. This smaller format is 22.7mm x 15.1mm, and is known as the APS-C format. Sony and Nikon (and others) use a sensor size that is approximately the APS-C format size. As can be seen, the APS-C size provides a much smaller image area, 342.77 square millimeters versus 864 square millimeters for the standard 35mm format.

When a 35mm lens is used on the current crop of digital cameras, the smaller sensor size means that the 35mm lens will now have a narrower angle of view (also called FOV or field of view) than when used on a 35mm film camera body. This is due to the fact that the 35mm lens creates an image on the film (or sensor plane) that is is sufficiently large enough to cover the 36x24mm frame it is designed for.

When used in a Digital SLR that has a smaller sensor (and thus image size), the image formed by the 35mm lens is still the exact same size as before, but the smaller sensor can only use a portion of it. In effect, this constitutes an “in-camera” crop. This cropping is determined by the ratio of the sensor size to the 35mm format size, distance of the image plane from the front of the lens etc.

In most of the cameras using APS-C size sensors, the ‘cropping’ is equivalent to using a lens that has a narrower angle of view (telephoto effect). This Crop Factor (also known as Focal Length Multiplier) is around 1.5x (Sony Alpha) 1.6x (Nikon). In some cameras, Canon uses a sensor size that is slightly larger, which gives a 1.3x factor (this is the APS-H format). The Four-Thirds system used by Olympus and Lumix (Panasonic) has a 2x factor.

The focal length does not really change, of course. The smaller sensor’s ability to register only a portion of the total image causes an APPARENT increase in focal length due to the cropping. This has the effect of using a lens with a narrower angle of view, the same as using a telephoto lens. (Luckily, the sweet center portion has the sharpest part of the image).

The image edges which may not be sharp, or have aberrations are cropped in-camera. For the user, it’s easier to remember by multiplying the focal length of the lens they are using with the factor for that particular body, and that gives us the APPARENT focal length.

A lot of the confusion can be eliminated by using the term APPARENT rather than EQUIVALENT. for example, on my Sony Alpha 700 (which has a 1.5x multiplier for 35mm lenses), it would be more accurate to say that my Minolta 50mm lens has an APPARENT focal length of 75mm, rather than saying my Minolta 50mm is EQUIVALENT to 75mm.

This apparent increase in focal length does not affect the aperture, so the APPARENT focal length of 75mm is still at a fast f/1.7. This will serve as an excellent portrait lens, since it can frame head and shoulders perfectly, but you’ll find yourself having to move back when you want to include more of the scene.

For most wildlife photographers, birdwatchers and even landscape photographers, the apparent increase in focal length can be a unexpected blessing – they get more ‘bang’ for the buck from their existing lenses. When coupled with the higher ISO capabilities of DSLR cameras and in-camera image stabilization that will let them handhold the camera in situations that would have required them to lug along a heavy tripod.

If you’re using a MACRO lens, such as, for example my Sigma 50mm f/2.8 (a very sharp lens with a true 1:1 macro capability) the apparent increase in focal length permits the macro subject to be fill the frame without moving in as close. Since the stand-off distance is greater, more light can reach the subject. The increased distance also helps when photographing skittish subjects that are likely to fly or hop away if approached too closely.

When the subject is framed in the Sony Digital SLR, it is seen that the Depth of Field is greater for the same magnification when compared to using the lens on a 35mm film camera like my Minolta Dynax 800si. This is a great advantage for macro-photography where depth of field and subject lighting are always problematic.

The whole situation is different when it comes to wide angle lenses. This is where most photographers are ready to burst into tears. The 1.5x or 1.6x multiplier effect holds good on the wide angles too, and this means that a normal wide angle like 24mm lens which would give a 74 degree field of view on a 35mm format camera will now only provide a FOV of 62 degrees or so, approximately that of a 35mm lens. In the same way, an ultra-wide 16mm becomes an apparent 24mm a 28mm is an apparent 42mm and a 35mm lens behaves like a 52.5mm normal lens.

Given the fact that most wide angle lenses are EXPENSIVE, no photographer would like to see his investment reduced to a moderate wide-angle or a normal lens. However, such is life. You win some, you lose some. To get a true wide angle on the Sony Alpha 700, I would have to invest in a 18mm to get 27mm (luckily, that’s included in the zoom range of my Sony 18-200mm lens that I bought with the camera). Since the 18-70mm is bundled as a kit lens, most owners will have at least a 27mm wide angle, albeit a slow one).

To get the 24mm, we’d have to invest in the very expensive Sony 16-105mm Zeiss coated zoom lens (apparent focal length 24- 157.5mm). Buying a faster wide-angle prime lens would be prohibitively expensive for any amateur, and even professionals would hesitate.

My advice? If you really need to shoot wide angle, just use your film camera. You can still buy a fine Sigma 24mm f/2.8 AF at a reasonable price. The Sigma 24mm is a good deal, since it had a matte black Zen finish that tended to flake off, giving the lens a “very used” appearance and usually marked down. This is just cosmetic, and does not affect the lens performance in any way.

The same Sigma lens was re-badged and sold by Ritz as their house-brand Quantaray lens, but with a different, more durable finish – but it’s still the same lens. Don’t be fooled by the appearance of either lens – this is a very high quality lens. Besides, you can always use the Sigma on the digital SLR as an apparent 35mm f/2.8 prime lens in place of a ‘Normal’ lens. You’d pay hundreds more to get a similar “designed for digital” Nikon, Sony or Canon lens.

Of course, all this talk about crop factors and multiplication factors will become moot when Full Frame digital SLRs become common. Canon already has one, and Nikon and Sony will follow shortly. In fact, I would not be surprised if the Sony Alpha 900 was not released in a few more months. When that happens, lenses will behave as they were designed and there will be no more confusion.

As for me, I rather like the fact that my Tamron 70-300mm now behaves like a 105mm-450mm super-zoom. With the Image stabilization turned on, I can actually hold that baby steady enough to use hand-held.